Abstract
The invention describes a method and an arrangement for transmitting an optical transmission signal with reduced polarization-dependent loss. A first transmission signal component and a second orthogonal transmission signal component of the optical transmission signal are transmitted with a time difference between said transmission signal components.
Claims
1. Method for transmitting an optical transmission signal Z(t) with reduced polarisation-dependent loss comprising: generating a first transmission signal component Z.sub.U(t) and a second transmission signal component Z.sub.V(t) of the optical transmission signal Z(t) wherein Z.sub.V(t) and Z.sub.U(t) are orthogonal, using the following pre-delaying step wherein D is a time difference between said first and second transmission signal components Z.sub.U(t), Z.sub.V(t), wherein u(t) and v(t) are derived or derivable from orthogonal signals S.sub.X(t) and S.sub.Y(t) by a step of rotation according to the following relationship: where S.sub.X(t) and S.sub.Y(t) are signals that are orthogonally polarisation multiplexed and modulated according to first and second data signals, respectively, and R is a rotation matrix representing a rotation by a fixed angle, wherein said rotation and said pre-delaying are carried out either in the optical domain, or in the electrical or digital domain, by corresponding differential group delay predistortion component modulation signals for generating said transmission signal components Z.sub.U(t), Z.sub.V(t) directly by modulating allocated carriers, and transmitting the optical transmission signal Z(t) with the time difference of D between Z.sub.U(t) and Z.sub.V(t).
2. The method according to claim 1, wherein the first and second transmission signal components of the transmission signal are transmitted with equal power.
3. The method according to claim 1, wherein the first and second transmission signal components are transmitted with a time delay of one half of a modulation section or one half of a modulation section and an integer multiple of a modulation section between the orthogonal transmission signal components.
4. The method according to claim 1, wherein the optical transmission signal is amplitude, quadrature amplitude, frequency or phase modulated, or a combination thereof, or an orthogonal frequency division multiplex signal.
5. The method according to claim 1, wherein the first transmission signal component and the second transmission signal component of the transmission signal are generated by differential group delay (DGD) predistorted electrical component modulation signals derived from component modulation signals.
6. The method according to claim 1, wherein the method further comprises coding a first data signal to yield a first pair of values of a first complex value and coding a second data signal to yield a second pair of values of a second complex value; the complex values are converted into (differential group delay) DGD pre-distorted component modulation values, wherein said DGD pre-distorted component modulation values are converted into DGD pre-distorted electrical component modulation signals, and the first transmission signal component and the second transmission signal component of the optical transmission signal are generated by said electrical DGD pre-distorted component modulation signals.
7. The method according to claim 1, wherein at a receiver the time difference between orthogonal transmission signal components of a received optical polarisation multiplex transmission signal is compensated to achieve reconstructed polarisation multiplex signal components.
8. The method according to claim 7, wherein the received polarisation multiplex signal transmission signal is coherently demodulated and sampled, and the time difference of the orthogonal signal components represented by complex data signal values is compensated.
9. The method according to claim 8, wherein a butterfly filter is used for compensation of the time difference between the orthogonal transmission signal components of the transmission signal and for reconstructing transmitted data signals.
10. The method of claim 1, wherein said fixed rotation angle is an angle of 45°, and said rotation matrix R is defined as wherein C is a constant.
11. An arrangement for transmitting an optical signal with reduced polarisation-dependent loss comprising: a transmission signal component generator for generating a first transmission signal component Z.sub.U(t) and a second transmission signal component Z.sub.V(t) of the optical transmission signal Z(t) wherein Z.sub.V(t) and Z.sub.U(t) are orthogonal, using the following pre-delaying step wherein D is a time difference between said first and second transmission signal components Z.sub.U(t), Z.sub.V(t), wherein u(t) and v(t) are derived or derivable from orthogonal signals S.sub.X(t) and S.sub.Y(t) by a step of rotation according to the following relationship: where S.sub.X(t) and S.sub.Y(t) are signals that are orthogonally polarisation multiplexed and modulated according to first and second data signals, respectively, and R is a rotation matrix representing a rotation by a fixed angle, wherein said rotation and said pre-delaying are carried out either in the optical domain, or in the electrical or digital domain, by corresponding differential group delay predistortion component modulation signals for generating said transmission signal components Z.sub.U(t), Z.sub.V(t) directly by modulating allocated carriers, and a combiner for combining the first transmission signal component Z.sub.U(t) and the second transmission signal component Z.sub.V(t) to produce the optical transmission signal Z(t) with the time difference D between Z.sub.U(t) and Z.sub.V(t).
12. An arrangement for receiving a signal Z(t) transmitted by the arrangement of claim 11, comprising: means for constructing data values; and means for reconstructing said first and second data signals.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Principles and presently preferred examples of the invention are described below with reference to accompanying drawings, where
(2) FIG. 1 shows a first simplified diagram of a transmitter with an optical delay,
(3) FIG. 2 shows a first simplified diagram of a receiver with an optical delay,
(4) FIG. 3 shows a second simplified diagram of a transmitter with an electrical delay,
(5) FIG. 4 shows a third simplified diagram of the transmitter with a PMD emulator,
(6) FIG. 5 shows a block diagram of a QAM transmitter with an optical delay,
(7) FIG. 6 shows a simplified block diagram of a general polmux transmitter,
(8) FIG. 7 shows a polarisation diagram of optical signals at the polmux transmitter,
(9) FIG. 8 shows a block diagram of a QAM polmux transmitter with an electrical DGD pre-distortion,
(10) FIG. 9 shows a block diagram of a QPSK polmux transmitter with a digital DGD pre-distortion,
(11) FIG. 10 shows a diagram of a polmux receiver, and
(12) FIG. 11 shows the performance improvement of the proposed method.
(13) FIG. 1 illustrates a simplified block diagram of a transmitter comprising a modulation unit 2 and a signal DGD pre-distortion unit 3-5 (DGD—differential group delay). A data signal DS (or a modulation signal derived from DS) is fed to an input 1 of the modulation unit 2 and modulated onto an optical carrier CW (continues wave) connected to input 7 of the modulation unit. The modulated optical signal s(t) is split by a polarisation beam splitter 3 into two orthogonal signal components k.sub.X and k.sub.Y (assuming linear polarisation and optimal performance, if the polarisation angle of s(t) is e.g. 45°, k.sub.X has a polarisation angle of 0°, k.sub.Y has a polarisation angle of 90°, and both signals amplitudes have equal power; non ideal splitting results in poorer performance). One of these signal components, e.g. k.sub.Y, is delayed by a delay time “D” by an optical delay element 4. A delay of ¼ of a modulation section leads already to a considerable improvement, and a delay time about ½ of a modulation section leads to a very good result. Advantageously are also delay times of 1.5, 2.5, . . . modulation sections. The transmission signal component k.sub.X(t) and the delayed orthogonal transmission signal component k.sub.Y(t) are then combined by a combiner (splitter) 5 to a transmission signal k(t) and transmitted via a transmitter output 6 (the time variable “(t)” is sometimes omitted in the drawings).
(14) The DGD pre-distortion is compensated at the receiver by means of static DGD compensation elements or by existent adaptive filters. An appropriate receiver is shown in FIG. 2. A polarisation controller 9 receives the transmission signal k(t) and an output signal is fed to a polarisation beam splitter 10 which splits it into the orthogonal signal components k.sub.X(t) and k.sub.Y(t) (the same signals output by the transmitter; therefore the same reference signs are used for the transmitter and receiver signals). Now the other signal component k.sub.X(t) is delayed by an optical delay element 11, and then combined by a polarisation beam combiner 12 with the signal component k.sub.Y(t) (delayed in the transmitter). The regained signal s(t) is demodulated by a demodulation unit 13. The data signal DS is output at terminal 14.
(15) FIG. 3 shows another embodiment of a transmitter with two modulators 21 and 22, both receiving carrier signals CW1, CW2 derived by a splitter 5 (or polarisation beam splitter) from the same source generating a constant wave signal CW. Both carriers are separately modulated by a modulation signal p(t) derived from the data signal DS, but the modulation signal fed to modulator 21 is delayed by an electrical delay element 23. The modulated orthogonal signal components k.sub.X(t) and k.sub.Y(t) are combined by a polarisation beam combiner 12 to the transmission signal k(t). In all arrangements, a polarisation beam splitter/combiner may be substituted by a splitter/combiner and vice versa. Of course the polarisations of the signal components have to be considered and, if necessary, corrected.
(16) FIG. 4 shows a PMD-emulator (PMD—polarisation mode dispersion) 15 generating the transmission signal k(t). A modulated light beam LI corresponding to the modulated optical signal s(t) is fed to the emulator made of a birefringent material, e.g. lithium niobate. The orthogonal components of the light beam LI are delayed by different amounts and the transmission signal k(t) comprising the orthogonal signal components k.sub.X(t) and k.sub.Y(t) is output. The delay time is controllable by applying an electrical field.
(17) FIG. 5 shows a more detailed diagram of a transmitter generating a QAM-signal (quadrature amplitude modulation). A data source 16 generates the data signal DS which is fed to a serial/parallel converter 17. The parallel data bits b.sub.1-b.sub.m are optionally coded in e.g. differential coders—only one coder 18 is shown—and then coded in a QAM-coder 19. A level generator 20 outputs complex data values, in this embodiment separately real values I (in phase values) and imaginary values Q (quadrature values). The signal values I, Q are converted into electrical modulation signals p.sub.I(t) and p.sub.Q(t) by signal converters 24, 25 and fed to optical modulators 31 and 32 respectively. Optical carrier signals CW1, CW2 are generated by a CW generator (laser) 26, in this embodiment shaped by an impulse carver 27, split and then fed to the modulators 31 and 32. One of the carrier signals or one of the modulated signals is phase shifted by a phase shifter 30. The modulated signals output by the modulators are combined to a modulated optical signal s(t) (the same reference signs are used for equivalent signals relating to the invention). This signal s(t) is then treated in the same way as shown in FIG. 1. First, s(t) is rotated by a polarisation rotator 66 (if not already done by rotating the components). Then s(t) is split by the polarisation beam splitter 3 into two orthogonal signal components according FIG. 1 (both orthogonal signal components having preferable equal power), one signal component k.sub.Y(t) is delayed and then both signal components k.sub.X(t), k.sub.Y(t) are combined to the transmission signal k(t). This figure shows the application of the invention in combination with a real transmitter. The described method may be also applied when k(t) is a polarisation multiplex signal s.sub.X(t)+s.sub.Y(t) (s.sub.X, s.sub.Y—orthogonal vectors) shown in FIG. 7.
(18) FIG. 6 shows a simplified block diagram of a digital polmux transmitter. The transmitter comprises a first coder 35 and a second coder 36. The data signal DS is split into a first data signal DSX coded by the first coder 35 and a second data signal DSY is coded by the second coder 36.
(19) Complex signal values X.sub.I, X.sub.Q are output by the first coder and complex signal values Y.sub.I, Y.sub.Q are output from the second coder. The complex signal values are converted by a conversion unit 37 into DGD pre-distortion modulation signals r.sub.UI, r.sub.UQ and r.sub.VI, r.sub.VQ; both pairs of complex modulation signals modulating the allocated optical carrier CW. The DGD pre-distortion may be executed before or after the conversion into electrical modulation signals.
(20) FIG. 7 shall explain how an adequate polmux transmission signal z(t) is generated. It is assumed that s.sub.X(t) and s.sub.Y(t) are orthogonal modulated optical signals generated by a standard polmux system. These signals s.sub.X(t) and s.sub.Y(t) are converted into a new pair of orthogonal component signals, a “vertical” component signal u(t) and an orthogonal “horizontal” component signal v(t) (of course the coordinates are arbitrarily).
(21) Assuming linear polarisation of the orthogonal signals s.sub.X(t) and s.sub.Y(t), these signals are preferable rotated preferable by 45° to receive optimized performance and their vertical (u-polarisation) and their horizontal (v-polarisation) polarized component signals are constructed according to
(22)
or generated by an equivalent arrangement; C—constant value. Then one of the new orthogonal component signals u(t) or v(t), each comprising signal components of the signals s.sub.X(t) and s.sub.Y(t) with the same polarisation, is pre-delayed
(23)
and the orthogonal “transmission signal components” z.sub.U(t) and z.sub.V(t) are combined to a polarisation multiplex transmission signal z(t) according to our invention. This mathematical operation may be executed in the optical domain using commercially availably components. But it is advantageous to apply this method in the electrical or digital domain generating corresponding DGD pre-distortion component modulation signals. The component modulation signals generate the transmission signal components z.sub.U(t), z.sub.V(t) directly by modulating allocated carriers.
(24) If DGD pre-distorted modulation signals are used, then the orthogonal transmission signals s.sub.X(t) and s.sub.Y(t) do not really exist at the transmitter side, but they are enclosed in the transmission signal components z.sub.U(t), z.sub.V(t) of the polmux transmission signal z(t). At the receiver side, the standard polmux signals or corresponding demodulated signals have to be reconstructed to regain the data signal.
(25) According to the FIGS. 1-6 many different arrangements may be used for generating a DGD pre-distorted transmission signal. The invention is independent of coding or modulation, e.g. QAM, OFDM, PSK, DPSK, ASK, or combinations thereof may be applied for single polarisation and polarisation multiplex signals.
(26) It is of course advantageous to generate the pre-delayed polmux signal by manipulating the electrical modulation signals or even by digital calculations.
(27) FIG. 8 shows a more detailed polmux transmitter block diagram with electrical DGD pre-distortion. The data signal DS is split into two groups of parallel signals DSX=b.sub.1X-b.sub.mX and DSY=b.sub.1Y-b.sub.mY. The first group DSX is separately coded in a coding arrangement 18-20 shown in FIG. 5 and described before. The second group DSY is coded in a second identical coding arrangement 42-44. Each group of parallel signals is output as a pair of complex values X.sub.I, X.sub.Q and Y.sub.I, Y.sub.Q and converted into electrical modulation signals p.sub.XI, p.sub.XQ and p.sub.YI, p.sub.YQ by signal converters 24, 25 and 45, 46 respectively. The electrical modulation signals are DGD pre-distorted and converted into component modulation signals r.sub.UI(t), r.sub.UQ(t) and r.sub.VI(t), r.sub.VQ(t) by the DGD pre-distortion unit 41 according the equations (1) and (2). The first orthogonal transmission signal component z.sub.U(t) is generated by the upper modulator 38 and the second orthogonal transmission signal component z.sub.V(t) is output from the lower modulator 39. These signals are combined to the polmux transmission signal z(t).
(28) FIG. 9 shows another embodiment of a polmux transmitter with digital DGD-pre-distortion. A digital DGD pre-distortion unit 47 is inserted between the level generators 20, 44 and the signal converters 24, 25 and 45, 46. The signal values X.sub.I, X.sub.Q and Y.sub.I, Y.sub.Q output by the level generators 20 and 44 are converted into component modulation values RUI, RUQ and RVI, RVQ according the equations (1) and (2) by digital calculation and then converted into the component modulation signals r.sub.UI(t), r.sub.UQ (t) and r.sub.VI(t), r.sub.VQ(t). The component modulation signals generate the orthogonal transmission signal components z.sub.U(t) and z.sub.V(t).
(29) FIG. 10 shows a coherent receiver for a polmux system which is transmitting two orthogonal optical signals z.sub.u(t) and z.sub.V(t) according to the invention. The arrangement of the receiver is well known to those skilled in the art. The receiver is implemented for coherent demodulation to allow digital processing of complex sample values and therefore the reconstruction of the original information independent of the polarisation of the received signals. The transmission signal z(t) (ignoring impairments) is received at the input 50 of a polarisation beam splitter 51. The transmission signal comprises the orthogonal optical transmission signal components z.sub.U(t) and z.sub.V(t) which carry information derived from the data signals DSX and DSY (FIG. 8, FIG. 9). Because the transmission signal z(t) is received with an unknown polarisation it is split into a x-polarisation signal z.sub.X(t) and an orthogonal y-polarisation signal z.sub.Y(t). Both signals are fed to 90°-hybrids 52 and 53 respectively. The x-polarisation 90°-hybrid outputs a real and an imaginary x-polarisation component signal z.sub.XI, z.sub.XQ and the y-polarisation 90°-hybrid outputs a real and an imaginary y-polarisation component signal z.sub.YI, Z.sub.YQ. These signals are converted into electrical signals, sampled and output by analogue-digital converters 54-57 as pairs of complex values Z.sub.XI, Z.sub.XQ and Z.sub.YI, Z.sub.YQ respectively. After passing chromatic dispersion compensation (CDC) 58 and 59 and a clock recovery 60, the complex component values Z.sub.X(n)=Z.sub.XI, Z.sub.XQ and Z.sub.Y(n)=Z.sub.YI, Z.sub.YQ (n=time instant) are fed to a four dimensional MIMO (multiple input-multiple output) filter 61, here denoted as butterfly filter, for reconstructing data values D.sub.X(n) and D.sub.Y(n) of the data signals DSX and DSY respectively. A carrier recovery 62 is necessary for control purposes. The symbol estimation unit 63 receives the complex data values D.sub.X(n) and D.sub.Y(n) and outputs the data signals DSX and DSY which are then multiplexed by a parallel-serial converter 64 to the data signal DS which is output at the receiver output 65.
(30) FIG. 11 shows the improvement for a QAM transmission system with DGD pre-distortion compared with SOP scrambling, assuming negligible nonlinearities in the optic channel, as well as a lumped additive white Gaussian noise source at the receiver.
(31) The present invention is not limited to the details of the above described principles. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalents of the scope of the claims are therefore to be embraced by the invention. Adequate mathematical conversions or adequate calculations and adequate and non ideal generation of the transmission signal components are also incorporated.
REFERENCE SIGNS
(32) 1 transmitter input 2 modulation unit 3 polarisation beam splitter 4 optical delay element 5 combiner 6 transmitter output 7 CW input 8 receiver input 9 polarisation controller 10 polarisation beam splitter 11 optical delay element 12 polarisation beam combiner 13 demodulation unit 14 receiver output 15 PMD emulator 16 data source 17 serial-parallel converter 18 differential coder 19 QAM coder 20 level generator 21 first modulation unit 22 second modulation unit 23 electrical delay element 24 signal converter 25 signal converter 26 CW generator 27 pulse modulator 28 pulse carver 29 splitter 30 phase shifter (delay) 31 modulator 32 modulator 33 combiner (splitter) 34 serial-parallel converter 35 first coder 36 second coder 37 DGD pre-distortion 38 first modulation unit 39 second modulation unit 40 (polarisation beam) combiner 41 electrical pre-distortion unit 42 differential coder 43 QAM coder 44 level generator 45 signal converter 46 signal converter 50 receiver input 51 polarisation beam splitter 52 x-polarisation hybrid 52 y-polarisation hybrid 54-57 analogue-digital-converter 58, 59 chromatic dispersion compensator 60 clock recovery 61 butterfly filter 62 carrier recovery 63 symbol estimation unit 64 parallel-serial-converter 65 receiver output 66 polarisation rotator DS digital data signal CW carrier signal (constant wave) D delay time s(t) modulated optical signal k.sub.X(t) x-polarisation signal component k.sub.Y(t) y-polarisation signal component k(t) transmission signal LI light beam XI real signal value (in phase) XQ imaginary signal value (quadrature) CW1 first carrier signal CW2 second carrier signal g(t) conversion function p.sub.I(t) modulation signal p.sub.Q(t) modulation signal DSX first data signal DSY second data signal X.sub.I real signal values of DSX X.sub.Q imaginary signal values of DSX Y.sub.I real signal values of DSY Y.sub.Q imaginary signal values of DSY RUI real u-component modulation value RUQ imaginary u-component modulation value RVI real v-component modulation value RVQ imaginary v-component modulation value p.sub.XI real x-component modulation signal p.sub.XQ imag. x-component modulation signal p.sub.YI real y-component modulation signal p.sub.YQ imag. y-component modulation signal r.sub.UI(t) real (u-polarisation) DGD pre-distorted component modulation signal r.sub.UQ(t) imaginary (u-polarisation) DGD pre-distorted component modulation signal r.sub.VI(t) real (v-polarisation) DGD pre-distorted component modulation signal r.sub.VQ(t) imaginary (v-polarisation) DGD pre-distorted component modulation signal s.sub.X(t) (x-polarisation modulated) signal s.sub.Y(t) (Y-polarisation) modulated) signal u(t) vertical signal component v(t) horizontal signal component z.sub.u(t) first orthogonal transmission signal component z.sub.v(t) second orth. transmission signal component z(t) polmux transmission signal z.sub.X x-polarisation component signal z.sub.Y y-polarisation component signal Z.sub.XI real x-polarisation component value Z.sub.XQ imaginary x-polarisation component value Z.sub.YI real x-polarisation component value Z.sub.YQ imaginary x-polarisation component value Z.sub.X(n) complex x-polarisation component value Z.sub.Y(n) complex y-polarisation component value n time instant
